1. Field of the Invention
This invention is in the field of ceramic electrical resistors based upon barium titanate and containing small amounts of one or more of the metals cobalt, chromium, nickel, vanadium, manganese and copper. The resistors of the present invention have low specific resistances below their Curie temperatures and, above their Curie temperatures, exhibit a rise in resistance of more than 4 and sometimes 5 powers of 10. Such resistors have a very steep resistance-temperature characteristic and evidence only a slight varistor effect.
2. DESCRIPTION OF THE PRIOR ART
In German Pat. No. 929,350 there is described a ceramic resistor with a positive temperature coefficient, based upon barium titanate, with an excess of titanium dioxide as well as an additive of silica. These materials were rendered semiconducting by doping them with metals such as bismuth, antimony or tungsten or rare earth such as yttrium, lanthanum, gadolinium or erbium. These resistors, above their Curie temperatures, exhibited an increase in resistance by a factor of 100 to 1,000.
In German patent application PS No. 1,471,445 there is described electrical resistors having a positive resistance temperature coefficient and a low dependence of resistance upon applied voltage. The resistors described in this application are based upon perowskite structures which have been doped by means of antimony, niobium or lanthanum. The rise in resistance above the Curie temperature is achieved by a specific distribution of trivalent and pentavalent antimony ions between the grain interior and the grain boundaries. Such a distribution is difficult to achieve as a practical process. Also, these resistors are moisture sensitive so that when used under normal operating conditions they have to be incorporated into moisture tight housings.
In German application AS No. 1,941,280 there is described a ceramic semiconductor with a resistance having a positive temperature coefficient based upon a material of perowskite structure with a silica additive and containing a manganese dopant, together with a rare earth, bismuth or antimony. This resistor, above its Curie temperature, exhibits a resistance increase of more than 4 powers of 10. This resistor has a weak varistor effect, that is, its resistance is not substantially dependent upon the applied voltage, although its resistance-temperature characteristic can be influenced by atmospheric effects.
In German application OS No. 1,465,109 there is described a resistor having a positive resistance temperature coefficient consisting of a material of perowskite structure based upon barium titanate or barium-strontium titanate or barium-lead-titanate, and having a titanium surplus and a small amount of tin oxide. The doping substances used to create the conductivity may be materials such as praseodymium, neodymium or samarium, or a mixture of the rare earths cerium, lanthanum, neodymium, praseodymium, samarium, gadolinium or yttrium. This resistor, above its Curie temperature, exhibits a rise in resistance of about 3 powers of 10.
In German applications OS No. 1,646,987 and OS No. 1,646,988 there are described electrical resistors having a positive resistance temperature coefficient and consisting of doped ferroelectric material of perowskite structure having the formula Me.sup.II Me.sup.IV O3 with a surplus of Me.sup.IV metals. The doping substances used in order to produce conductivity in this type of resistor may be antimony, niobium or lanthanum. The resistors may also contain copper or iron. The rise in resistance above the Curie temperature, in these resistors, is about 4 powers of 10.